1. Field of the Invention
The present invention relates to a heating apparatus that is suitable for a fixing apparatus for use in a dry-type electrophotographic apparatus; a drying apparatus for use in a wet-type electrophotographic apparatus; a drying apparatus for use in an ink jet printer; or an erasing apparatus for a rewritable medium. The invention also relates to an image forming apparatus employing said heating apparatus.
2. Description of the Related Art
A heating apparatus has hitherto been used as a fixing apparatus for fixing a toner image onto a recording sheet in an electrophotographic copier or printer, for example. In a conventional fixing apparatus, inside a heating roller having a hollow core metal made of aluminum or the like is arranged a halogen lamp. By driving the halogen lamp to liberate heat, the heating roller is heated to a predetermined temperature.
However, the above-described method using a halogen lamp poses a problem that a long warm-up time is required due to a sluggish rise of temperature upon starting of heating. To shorten the warm-up time, it can be considered that the thickness of the heating roller is reduced to decrease thermal capacity. However, a reduction in the thickness of the heating roller is necessarily accompanied by a decrease in rigidity. If the rigidity of the heating roller is low, the heating roller suffers from considerably large distortion when pressed by a pressure-applying roller, resulting in a decrease in a pressure-applying force exerted on the longitudinal central portion of the heating roller. This causes fixing failure. In the end, there is a limit to shortening of the warm-up time in accompaniment with a decrease in capacity achieved by reducing the thickness of the heating roller. In particular, in a fixing apparatus with a halogen lamp designed for use in a color image forming apparatus, outside a core metal is formed a 1 to 3 mm-thick elastic layer. Therefore, even if the thickness of the core metal is reduced to decrease thermal capacity, the elastic layer has large thermal capacity, and thus it takes much time for warming up. In addition, a reduction in the thickness of the elastic layer leads to fixing failure. In the end, there is also a limit to reducing the thickness of the elastic layer.
To solve the above-described problems associated with the conventional fixing apparatus, Japanese Unexamined Patent Publication JP-A 8-129313 (1996) discloses one prior art practice. The prior art disclosed in JP-A 8-129313 is as follows. There is provided a heating roller having an elastic layer formed inside. Outside the elastic layer is formed a 10 to 150 xcexcm-thick conductive layer. In the heating roller, the conductive layer is heated externally. Formed in the outer periphery of the conductive layer is a peeling layer. The features of the disclosed prior art will be described below.
Since the heating roller is formed of a thin conductive layer, its thermal capacity can be kept small, whereby making it possible to shorten the warm-up time. Moreover, the conductive layer has adequate rigidity and is securely formed on the elastic layer fixed onto the core metal. Thus, excellent durability can be attained in the heating roller. By providing an elastic layer inside the heating roller and by exploiting the elasticity of the pressure-applying roller, it is possible to increase flexibility in selecting a width of a nip portion where the heating roller and the pressure-applying roller make contact with each other. This makes it possible to achieve speedup in the operation of the image forming apparatus. The axial distortion of the pressure-applying roller is diminished by the elastic layer formed inside the heating roller, so that the longitudinal width of the nip portion is kept uniform. Hence, load on a material to be heated is made uniform and thus occurrence of a ripple or other troubles can be prevented. Moreover, the adequate rigidity of the conductive layer serves to make uniform the longitudinal width of the nip portion. By setting the surface hardness of the pressure-applying member to be equivalent to or higher than that of the heating roller, the nip portion can be made flat, thus preventing the to-be-heated material from curling up due to the curvature of the heating roller.
With the prior art disclosed in JP-A 8-129313, however, although the warm-up time can be shortened successfully, the surface of the heating roller is made undesirably hard, because the peeling layer is formed directly in the outer periphery of the conductive layer. An unfixed to-be-heated material has, on its top surface, irregular concavities and convexities ascribable to presence/absence of toner, or to difference in toner layer thickness. Therefore, when heat is applied to a toner image formed on the to-be-heated material by the heating roller having a hard surface, the heating roller fails to conform to the irregular concavities and convexities, resulting in occurrence of uneven heating. This leads to fixing failure and uneven gloss. In particular, when a color image formed by stacking together a plurality of toner images of different colors is subjected to fixing, it is impossible to obtain adequate fixability.
An object of the invention is to provide a heating apparatus that allows shortening of warm-up time and ensures excellent fixability and gloss property, and an image forming apparatus incorporating the heating apparatus.
The invention provides a heating apparatus comprising:
a heating rotary member including: a first elastic layer made of a material having elasticity; a conductive layer disposed in an outer periphery of the first elastic layer; a second elastic layer made of a material having elasticity, which is disposed in an outer periphery of the conductive layer; and a peeling layer disposed in an outer periphery of the second elastic layer;
a pressure-applying member disposed in press-contact with the heating rotary member, for allowing a sheet-like to-be-heated material to be conveyed by the heating rotary member and the pressure-applying member as nipped therebetween; and
heating means for applying heat to the conductive layer of the heating rotary member.
According to the invention, the heating rotary member is provided with the second elastic layer, which is made of a material having elasticity, formed in between the conductive layer disposed in the outer periphery of the first elastic layer and the peeling layer. This helps keep the surface of the heating rotary member from becoming hard, and thus allow the heating rotary member to be elastically deformed adequately. In this connection, assuming that, e.g. a toner image is formed on a to-be-heated material which is conveyed by the heating rotary member and the pressure-applying member as nipped therebetween. In this case, even if concavities and convexities are created on the surface of the to-be-heated material due to the presence of the toner image, since the surface of the heating rotary member conforms to the concavities and convexities, occurrence of uneven heating can be avoided. Consequently, it is possible to secure a wide non-offset region, i.e., a fixing temperature range such as to obtain a high-quality fixed image in which toner is molten sufficiently and is thus no longer peeled off.
In the invention, it is preferable that a tensile elasticity modulus of the peeling layer is kept in a range from 1.96xc3x97108 Pa to 9.8xc3x97108 Pa.
If the tensile elasticity modulus of the peeling layer exceeds 9.8xc3x97108 Pa, the peeling layer becomes so hard that the heating rotary body fails to conform to the concavities and convexities completely, resulting in occurrence of uneven heating. For example, when a to-be-heated material having a toner image formed thereon is subjected to heating, toner fixing failure or uneven gloss occurs inevitably. On the other hand, if the tensile elasticity modulus of the peeling layer is less than 1.96xc3x97108 Pa, the peeling layer becomes too soft. Thus, although the heating rotary body is able to conform to the concavities and convexities on the to-be-heated material completely, it is impossible to obtain a sufficient effect of infiltrating toner into the to-be-heated material by melting. Consequently, excellent gloss property cannot be attained. Hereupon, according to the invention, by keeping the tensile elasticity modulus of the peeling layer within the above-described range, adequate elasticity can be imparted to the heating roller. Consequently, toner is allowed to infiltrate into the to-be-heated material while the heating rotary member conforms to toner concavities/convexities to some extent, whereby making it possible to obtain sufficient gloss property.
In the invention, it is preferable that the second elastic layer has a thickness in a range from 50 to 300 xcexcm.
If the thickness of the second elastic layer exceeds 300 xcexcm, the thermal capacity is increased, with the result that a long warm-up time is required. On the other hand, if the thickness of the second elastic layer is less than 50 xcexcm, the second elastic layer fails to provide sufficient elasticity, with the result that the surface of the heating rotary member no longer conforms to the concavities and convexities on the to-be-heated material. Hereupon, according to the invention, by keeping the thickness of the second elastic layer within the above-described range, it is possible to shorten the warm-up time and to prevent occurrence of uneven heating.
In the invention, it is preferable that the peeling layer has a thickness in a range from 5 to 50 xcexcm.
The peeling layer is located on the surface of the heating rotary member and brought into contact with the to-be-heated material. In view of this, if its thickness is less than 5 xcexcm, the durability becomes insufficient. On the other hand, if the thickness of the peeling layer exceeds 50 xcexcm, the elastic effect exerted by the second elastic layer, formed on the lower part of the peeling layer, is cancelled out, and thus the surface of the heating rotary member becomes hard. This leads to uneven heating. Hereupon, according to the invention, by keeping the thickness of the peeling layer within the above-described range, it is possible to realize a highly-durable heating apparatus free from uneven heating.
In the invention, it is preferable that the conductive layer has a thickness in a range from 10 to 100 xcexcm.
If the thickness of the conductive layer exceeds 100 xcexcm, the rigidity of the conductive layer becomes so high that, despite the presence of the elasticity of the first elastic layer located below the conductive layer, it is impossible to obtain an effect of forming a distortion jointly with the pressure-applying member. Consequently, the to-be-heated material having passed through a region between the heating rotary member and the pressure-applying member falls off upwardly. Moreover, it is impossible to secure an adequate width in the nip portion where the heating rotary member and the pressure-applying roller make press-contact with each other. Further, since the thermal capacity is increased, a long warm-up time is required. On the other hand, if the thickness of the conductive layer is less than 10 xcexcm, the durability is deteriorated, resulting in a breakage of the conductive layer. Moreover, since the thermal capacity is decreased, the to-be-heated material is subjected to considerable heat radiation. Consequently, in the case of conveying to-be-heated materials one after another, the heat radiation outruns the heating action. Hereupon, according to the invention, by keeping the thickness of the conductive layer within the above-described range, it is possible to obtain adequate rigidity and excellent durability. In addition, since the thermal capacity is kept within an appropriate range, the warm-up time can be shortened.
In the invention, it is preferable that the second elastic layer is made of silicone rubber.
According to the invention, by forming the second elastic layer from silicone rubber, excellent heat resistance can be attained. Moreover, since the conformability with respect to the concavities and convexities created on the surface of the to-be-heated material is improved, sufficient fixability can be secured.
In the invention, it is preferable that the peeling layer is made of a fluorine material.
According to the invention, by forming the peeling layer from a fluorine material, it is possible to obtain satisfactory peeling property for the surface of the heating rotary member and the toner deposited on the to-be-heated material. Moreover, the gloss property of the toner fixed onto the to-be-heated material can be enhanced to a sufficient degree.
In the invention, it is preferable that a surface roughness of the peeling layer is set at 0.3 xcexcm or below in terms of average surface roughness on the center line Ra, or set at 1.0 xcexcm or below in terms of ten point average surface roughness Rz.
If the center-line average surface roughness Ra, representing the surface roughness, exceeds 0.3 xcexcm, or the ten point average surface roughness Rz exceeds 1.0 xcexcm, a non-offset region cannot be secured sufficiently. Hereupon, according to the invention, by keeping the surface roughness within the above-described range, it is possible to secure a sufficiently wide non-offset region.
In the invention, it is preferable that the heating means is induction heating means for generating induced currents by applying an alternating magnetic field to the conductive layer, and the conductive layer liberates heat in the alternating magnetic field.
According to the invention, used as the heating means is the induction heating means for generating induced currents by applying an alternating magnetic field to the conductive layer. Thus, the structure of the heating roller can be simplified, and further the heating roller can be heated uniformly in a short period of time.
The invention further provides an image forming apparatus comprising:
a heating apparatus including: a heating rotary member having a first elastic layer made of a material having elasticity, a conductive layer disposed in an outer periphery of the first elastic layer, a second elastic layer, made of a material having elasticity, disposed in an outer periphery of the conductive layer, and a peeling layer disposed in an outer periphery of the second elastic layer; a pressure-applying member disposed in press-contact with the heating rotary member, for allowing a sheet-like to-be-heated material to be conveyed by the heating rotary member and the pressure-applying member as nipped therebetween; and heating means for applying heat to the conductive layer of the heating rotary member;
a visible image forming unit for forming a toner image on a to-be-heated material; and
conveying means for conveying a to-be-heated material having a toner image formed thereon in a region between the heating rotary member and the pressure-applying member.
According to the invention, the image forming apparatus includes the above stated heating apparatus. Thus, with the image forming apparatus, it is possible to form a high-quality image in correspondence with a wide range of setting temperatures of the heating apparatus.
According to the invention, the surface of the heating rotary member is kept from becoming hard, and thus the heating rotary member can be elastically deformed adequately. In this connection, assuming that a toner image, for example, is formed on a to-be-heated material which is conveyed by the heating rotary member and the pressure-applying member as nipped therebetween. In this case, even if concavities and convexities are created on the surface of the to-be-heated material due to the presence of the toner image, since the surface of the heating rotary member conforms to the concavities and convexities, occurrence of uneven heating can be avoided. Consequently, it is possible to secure a wide non-offset region, i.e., fixing temperature range of obtaining a high-quality fixed image in which toner is molten sufficiently and is thus no longer peeled off. Moreover, toner is allowed to infiltrate into the to-be-heated material properly so as to obtain sufficient gloss property. Further, the conductive layer to be heated is made thin-walled, and the heating rotary member is heated by exploiting an alternating magnetic field. This helps shorten the warm-up time. Hence, an energy-efficient, energy-saving fixing apparatus can be realized.
According to the invention, the image forming apparatus employs a heating apparatus having a wide non-offset region. Thus, with the image forming apparatus, it is possible to form a high-quality image in correspondence with a wide range of setting temperatures of the heating apparatus.